The invention relates to the field of pearlescent pigments. Pearlescent pigments, which simulate the effect of natural pearl, comprise platelet substrates, such as mica, coated with single or multiple layers of metal oxides or mixed metal oxides with high refractive indexes. The optical nature of these pigments is that each interface between two layered media with different refractive indexes allows partial reflection and partial transmission of incident white light. The light beams reflected (or transmitted) at the interfaces recombine constructively or destructively, depending on the phase relationship of these reflected beams, and result in intensity enhancement only for certain wavelengths. As a result, color is observed corresponding to these enhanced wavelengths at the specular angle. Depending on the optical thickness (physical thickness.times.refractive index) of the metal oxide(s) layer, beams in different range or ranges of wavelengths in the visible region undergo constructive interference and are observed as different colors. The higher the refractive index of the coating material, the higher the beam intensity reflected at the interfaces and the greater the luster and color effect observed.
The most widely used coating material on platelet substrates is titanium dioxide because of its high refractive index. The most widely used substrate is mica flakes having a high aspect ratio prepared by a wet grinding process. Generally, the titanium dioxide coating is accomplished by controlled hydrolysis of a titanium salt, commonly titanium oxychloride solution, and simultaneous deposition of the hydrous TiO.sub.2 particles formed by the hydrolysis onto substrate flakes which are suspended in the hydrolysis system. After calcination, the hydrous TiO.sub.2 layer transforms into highly reflecting TiO.sub.2, either in anatase or rutile phase. (See e.g. U.S. Pat. Nos. 3,087,828, 3,418,146, and 3,437,515).
A TiO.sub.2 layer with a rutile phase structure is generally preferred because of its higher refractive index, which results in stronger luster and color effects, and its higher stability in outdoor weathering. However, the surfaces of many substrates, including mica surfaces, favor the crystallization of the anatase phase structure. Direct deposit of hydrous TiO.sub.2 onto these flakes generally results in an anatase coating even though the product is calcined at temperatures as high as 900.degree. C. In order to produce rutile TiO.sub.2 coating layers, it is necessary to precoat the surface of the mica with a thin layer of hydrous SnO.sub.2 as a rutile initiator (See e.g. U.S. Pat. Nos. 4,038,099 and 4,086,100). A precoating of hydrous Fe.sub.2 O.sub.3 together with a small amount of Zn, Ca and/or Mg species has also been demonstrated to promote the rutile structure (See U.S. Pat. No. 5,433,779).
Although both SnO.sub.2 and Fe.sub.2 O.sub.3 can be used to produce highly lustrous interference pigments, there are disadvantages that limit the use of such pigments. Added SnO.sub.2 is not permitted in plastics for food contact applications or in cosmetic products in some countries. Additionally, Fe.sub.2 O.sub.3 has an inherent yellowish color, even in very small amounts, which can cause discoloration of certain products, especially white products. Consequently, other rutile promoting materials devoid of these limitations are highly desirable in order to achieve better product quality and broader applications.
U.S. Pat. No. 4,623,396 describes a process of coating mica particles with a first coating of a titanium compound and a second coating of titanium dioxide on the first coating. The titanium deposited in the first coating is a low oxide of titanium and/or a titanium oxynitride. However, the X-ray diffraction pattern of the first coating shows that the titanium dioxide in the first coating has the anatase structure.